This invention relates to a thick film thermal printing head suitable for reducing the size of a thermal transfer facsimile, thermal transfer printers or the like and for improving printing performance of such apparatuses.
A thick film thermal printing head, such as the one disclosed in Japanese Patent Unexamined Publication No. 60-232975, is known which has a structure in which a heating resistor is formed in close proximity to an end of a substrate. In this thick film thermal printing head, as shown in FIG. 3, a common electrode to which a plus voltage is applied from a power source is formed on an insulating interlayer formed on lead wires led from the heating resistor and connected to a driver IC and other lead wires led from the heating resistor and connected to the common electrode, and the common electrode in the form of a layer laid on the insulating interlayer and the lead wires which are located below the insulating interlayer and which are to be connected to the common electrode are electrically connected via through holes 15 formed in the insulating interlayer.
However, it is difficult to form through holes necessary for forming this structure by using the ordinary thick film manufacture process. For example, in the case of a thick film thermal head having a resolution of 8 pixels per 1 mm, it is necessary to form about 45 .mu.m square through holes in the insulating interlayer. A photolithography process is required to form such through holes because it is difficult to form them by using a thick film manufacture process based on screen printing alone, and the overall manufacture process is thereby made complicated.
According to the above-described prior art, the common electrode to which a plus voltage is applied and the driver IC are formed on the same side of the heating resistor, the lead wires led from the heating resistor and connected to the common electrode are formed on the substrate formed of a ceramic or a metal or on a glass glaze layer formed on the substrate, and the common electrode is formed on the insulating interlayer formed of glass and provided on the lead wires led from the heating resistor. The common electrode and the lead wires are connected via through holes formed in the insulating interlayer. However, it is difficult to reduce the size of each through hole to 100 .mu.m square or smaller if the layer forming process is based on screen printing and firing alone, and it is difficult to apply the above-described prior art to thermal transfer facsimile printing in conformity with a facsimile standard GIII mode having a resolution of 8 pixels per 1 mm necessary for forming about 45 .mu.m square through holes. To solve this problem, there is a need for a photolithography process for forming the through holes in the insulating interlayer. However, the number of process steps is thereby increased in comparison with the ordinary thick film process using screen printing and firing, so that the overall process is complicated and that the manufacture cost is increased.